1. Field of the Invention
The present invention relates to a method for manufacturing an SOI substrate having a silicon thin film on an insulative substrate.
2. Description of the Related Art
As a method for manufacturing an SOI substrate having a silicon thin film on an insulative substrate, there has been conventionally known a SmartCut method (e.g., Japanese Patent No. 3048201 or A. J. Auberton-Herve et al., “SMART CUT TECHNOLOGY: INDUSTRIAL STATUS of SOI WAFER PRODUCTION and NEW MATERIAL DEVELOPMENTS” (Electrochemical Society Proceedings Volume 99-3 (1999) p. 93-106).) for bonding a silicon substrate having hydrogen ions implanted on a bonding front surface side to a support substrate and then performing a heat treatment at approximately 500° C. or above to thermally delaminate a silicon thin film from a region where a concentration of the implanted hydrogen ions is maximum or an SiGen method (e.g., Specification in U.S. Pat. No. 6,263,941, Specification in U.S. Pat. No. 6,513,564 and Specification in U.S. Pat. No. 6,582,999) for performing a plasma treatment to bonding front surfaces of a silicon substrate having hydrogen ions implanted on a bonding front surface side and a silicon substrate or a substrate formed of any other material before bonding these substrates, then bonding both the substrates in a state where the front surfaces of these substrates are activated, carrying out a heat treatment at a low temperature (e.g., 100 to 300° C.) to increase bonding strength, and thereafter carrying out mechanical delamination at an ordinary temperature to obtain an SOI substrate.
In addition to such techniques, an “etch-back method” has been recently proposed as a technology of reducing a film thickness of a silicon layer of a bonded SOI substrate (see, e.g., Q. -Y. Tong, U. Goesele et al., “Semiconductor Wafer Bonding” Wiley (1998) Chapter 6. and others). According to this technique, boron (B) is ion-implanted or diffused in a front surface layer of a silicon wafer (a bond wafer) serving as an active layer to form a high-concentration p layer (a p++ layer) in which a B concentration is, e.g., 1019/cm3 or above (a specific resistance is approximately 0.01 Ωcm or below), and this wafer is bonded to a support substrate (a base wafer), and then the bond wafer is chemically etched from a back surface thereof to reduce a film thickness.
This etch-back method utilizes “etching selectivity” that an alkaline solution that is used to chemically etch a silicon crystal has a relatively high etching rate with respect to a silicon crystal having no boron added thereto or a silicon crystal having a general boron concentration but, on the other hand, it has a very low etching rate with respect to a silicon crystal having boron added thereto at a high concentration. It is to be noted that this “etching selectivity” (a selection ratio) is dependent on a boron concentration ratio in a crystal, but it usually falls within the range of 1 to 1000 and takes a higher value when a boron concentration ratio is increased.
However, since a silicon layer whose film thickness is reduced by the etch-back method has a very low resistance (a specific resistance of approximately 0.01 Ωcm or below), this resistance greatly deviates from a resistivity (a specific resistance of approximately 0.1 to 100 Ωcm) suitable for a general semiconductor device, and an application range cannot be help becoming narrow. Further, when a boron concentration of a p layer is adjusted to approximately 0.1 to 100 Ωcm in a specific resistance in order to set a resistivity of a silicon layer having a reduced film thickness to a resistivity suitable for a general semiconductor device, the above-explained “etching selectivity” (a selection ratio) cannot be sufficiently obtained, and uniformly acquiring a desired layer thickness in a substrate surface becomes difficult.
In view of such a problem, it is an object of the present invention to provide a method for manufacturing an SOI substrate which is superior in film thickness uniformity and resistivity uniformity of a silicon layer (an SOI layer) having a film thickness reduced by the etch-back method in a substrate surface.